Process of making precipitated calcium carbonate



Patented 15.1 .23, 1932 1 UNITED STATES.

PATENT 5 OFFICE JOHN WQG HURCH, Or ROSSLYN FARMS, PENNSYLVANIA, ANDHARVEY G. ELLEDGE,

OF PAINESVILIIE, OHIO, ASSIGNORS TO PURE CALCIUM PBDDUGTS COMPANY, 01'

n'rrsnuaen, PENNSYLVANIA, A coarom'rron or 01:10 rnocn'ss or namernncrr'rramn enem GABBONATE a no Draw1ng.

V 13 ant source of whiting is deposits of chalk in England and on theEuropean Continent. ghese chalk deposits were supposedly formed y thedeposition of great numbers of shells of microscopic organisms. Thechalk ther'efore is a very loosely coherent mass of. these tiny shellsand is easily broken up 1nto very small particle size. Disintegratedchalk has been referred to as amorphous, but the examination of minuteparticles of ,chalk under the ultra-microscope using polarlzed lightshows the unmistakablecrystalline structure .of calcium carbonate.

It has been supposed that thisso-called amorphous structure wasresponsible for all of the, desirable properties of chalk in the variousplastic arts. It has been definitely proven however, not only that" thechalk is not amorphous, but that one of the principal factorsresponsible for its desirable properties is the smallness of itsparticles. This fact, together with the fact that .the adsorptivecharacteristics of disintegrated chalk are low, has caused it to be usedto a very large extent in the plastic arts.

Various attempts have been made heretofore to duplicate chalk whitingbut they have resulted in little success. It has been attempted to grindlimestone but these attempts have failed because it is not possible byany method known to the art of grinding to break up limestone intoparticles quite as small as that of the chalk. It was known thatprecipitation, when properly controlled, would yield particle sizes evensmaller than those of the chalk, but precipitated whiting would not fillthe same need nor take the place of chalk because the surface conditionson the particles of the precipitated whiting were different from thesurface conditions on the particles of ground chalk. These equal enough,dition causes a lack of firmness in the bond Application filed December24,1928. Serial 1%. 828308. I

different surface conditions, which are indi-' cated by oil adsorptionvalues which diifer from the oil adsor tion values of ound chalk, areresponsib e for a peculiar ryness of the precipitated whiting ascompared with I chalk. Stated differently, it takes about twice as muchlinseed oil to make a putty from a given amount of the usualprecipitated whiting as it takes to produce a putty from an weight ofground chalk. Strangely to this same difference in surface conbetweenprecipitated calcium carbonate or whiting and the bonding material suchas oil or rubber. This results in a short putty, 66 I or a .short stifi,rubber stock which" is verydifiicult to calender.

Our invention rovides a process whereby the surface con precipitatedwhiting are changed as indi- 7o" cated by the lowering of the oiladsorption number, to give a product which whilehaving the advantage ofeven smaller particle size than ground chalk, possesses the otherphysical properties whlch render the chalk 76' so desirable. It has,been assumed until recently that the quantity of oil adsorbed by agiven weight of powder is inversely proportional 'to the particle sizeof the powder. In other words so the finer the particles in a given weiht of material of the same chemical analysls, the larger the surfacearea of the mass, and the greater the amount of oil adsorbed on thesurface of the particles. This assumption is true provided all otherconditions are the same. It has been found however that two powders ofidentical chemical analysis and of identical particle size, one havingbeen reduced 'to its size from a lar e mass bygrind- 00 ing, and theother having een brought to its particle size by precipitatiomhaveentireitions of the particles f For 9.3 1

ams of the powder for the latter. It will t 'erefore be evident that thesurface condicium carbonate and sodium tions of powders having the samechemical analysis and. the same particle size may differ, as indicated btheir oil adsorption values, because of di erent methods of producingthem.

One source material resulting from the production of sodium hydroxide.in an alkali plant by reaction between sodium carbonate and calciumhydroxide. The material resulting from this reaction contains bothlprecipitated cal-' ydroxide. The precipitated calcium carbonate may betreated in accordance with our invention to adapt it for use as a fillerfor plastics. Precipitated calcium carbonate after it' has beenfreedfrom impurities has an oil adsorption value of about 44 cc. r.100-grams of the powder. 'Ground chafir commonly used in the lastic artshas an oil adsorption value of a ut 22 cc. per 100 rams of powder. Byour invention .the' su ac'e conditions of the particles of precipitatedcalcium carbonate may be reduced oma value of about 44 cc. to a value ofabout 22 cc., so that it is adapted for use in the plastic arts.

In accordance with our invention the raw slurry or mud resulting fromthe reaction between'sodium carbonate and calcium hydioxide in thealkaliplant is'first washed to free the calcium carbonate from the so-111170 lime. The material coming from the rotary drier contains lumpswhich are next broken up, for example, by employing a roller millprovided with an air separator. This treatment in the roller mill onlybreaks up the lumps but does not a preciabl change the surface conditionon the particles.

' The finely divided particles coming from the roller mill are thensubjected to attrition,

for example, in a tube mill or'a ball mill. The halls employed in themill can be pebbles, iron rods, steels ugs, or steel balls, but weprefer to use case-hardened steel balls not exceeding inch in diameter.The time to which the material is subjected to attrition varies withinfairly wide limits. For example, if a small amount of material -weighingabout five pounds is attrited in a small ball mill, the attrition ispreferably carried on for about24 hours. On the other hand, if a largesize, continuous mill is employed the of precipitated whiting is the'charges on the v conditions being equal, the

longs to the class attritioncan be accomplished in a much shorter time.It should be understood that the attrition does not appreciabl'afiectthe sizes of the material particles. um carbonate particles in theslurry or mud r the calciobtained from the alkali (plant have a size ofabout 1 to microns lameter, the final product issuing from the mill inwhich the attrition is carried out will be of a size of about 1 to 5microns. It will thus be seen that the various stefis'ofthc' process donot cha e the size of t e precipitated particles but s m ly change theirsurface conditions so that t ey are better adapted to the proc for whichthey are to be used. The exact effact on the particles accomplished .by'the attrition .is not definitely knownbut it is believed that this stepchan 'esthe surface articles an ther'eby afl'ects the surface con ition,which is indicated by a change in 'oil' adsorption value.

According to a modified process, the attrition may be carried out whenthe recipitated material is in a wet condition. n this process the rawslurry or mud containing precipitated calcium carbonate, sodiumhydroxide and lime is treated'in the samamanner as previously describedin connection with the main process, in order to wash out the sodiumhydroxide and convert the lime into calciu m carbonate. The wet calciumcarbonate is then subjected to attrition in the'manner previouslydescribed, after which it is filtered and thendried at a temperaturenotexceeding 3009 F. After dr 'ng the lumps are disintegrated, for exampwhich the material is bolted or screened to produce the finishedproduct.Whether the attrition is carried out either in the d or the wet statethe size of the particles nee not be appreciably affected, toaccomplishthe'desired result. K

Whiting produced in accordance with our process is particularly adaptedfor use as a rubber filler. It weighs about 40 to pounds per cubic foot.Many of the wearing qualities of rubber when in use are gaged by theratio of stress to strain. In gleineral, all other of stress to strain,the better the resistance of the rubber to abrasion.

Pigments used in rubber are of three types namely, coloring pigments,reinforcing p1gments, and filhng fpigments. Whiting bef lling pigments.In this class. of pigments, the smaller the particle size of the ler,the higher the ratio of stress to strain. Since our whiting isprecipitated to a smaller particle size than chalk whiting can be groundeconomically, it gives -a higher ratio of stress to strain than an equalamount of ground chalk when used in equal ropor-. tion. However, if itwere attempts to use ordinary precipitated whiting as a filter forrubber, without treatment to change its sure in a cage mill, after gherthe ratio face conditions, the rubber would not have as great resistanceto abrasion as another rubber having the same proportion of groundchalk. The reason for this condition is that precipitated whiting whichhas not been treated to change the surface condition of the particlesdoes not have as good dispersion characteristics as ground chalk. Forthis reason precipitated whiting, without further treatment to changeits surface condition, is not as acceptacle for certain rubber uses asground chalk.

However, by changing the surface condition of precipitated calciumcarbonate par- ;ticles so that its oil adsorption value is as low asthat of ordinary ground chalk, a product which is superior to groundchalk is obtained. This is due to the fact that although the surfaceconditions of the precipitated Whiting and the ground chalk are thesame, the precipitated whiting particles are smaller than the particlesof the ground material and therefore gives a greater stress strain ratioand greater resistance to abrasion when used in rubber than doesordinary ground whiting.

We have described in detail two present preferred methods of carryingout our invention. It is to be understood however that the invention isnot so limited but may be otherwise embodied or practiced within thescope of the following claims.

We claim:

1. The process of making a filler for plastics, comprising precipitatingfinely divided calcium carbonate, and subjecting it to prolongedattrition for a time suflic1ent to reduce its oil adsorption value tobelow 25 cc. per grams of the calcium carbonate.

2. The process of making a filler for plastics, comprising precipitatingfinely divided calcium carbonate, and subjecting it to rolongedattrition for a time suflicient to re uce its oil adsorption toapproximately that of naturally occurrin ground chalk.

3. The process 0% making a filler for plastics, comp-rising precipitatmgfinely divided calcium carbonate, and subjecting it to rolongedattrition for a time sufiicient to re uce its oil adsorptionapproximately one-half.

In testimony whereof we have hereunto set our hands.

JOHN W. CHURCH. HARVEY G. ELLEDGE.

